Since it's about a boy and a horse, Andrew Haigh's Lean on Pete might at first lull you into a sort of audience complacency. The boy's broke, his dad's laid up and his equine pal Lean on Pete -- a low-rent racing quarter horse -- is starting to limp, which might spell its doom. Pete's owner, a cranky jumble of nerves and swears and resentments embodied by Steve Buscemi, isn't romantic about horses. His sometimes jockey (Chloe Sevigny) is a softer touch, but she reminds the boy, "It's not a pet." But the boy loves the beast, and if there's one thing that the movies have taught us, it's that no love is more sacred nor transformative than a young person's for an animal.

But writer-director Haigh (Weekend, 45 Years) dashes expectations in almost every scene. Working from a novel by Willy Vlautin, Haigh has committed himself to making a boy-and-his- horse movie that's scraped free of everything false or sentimental about the genre. Still, by the end, in its painstakingly observational way, Lean on Pete had devastated me twice and hung me there in a state of tense uncertainty for 30 or 40 minutes. Haigh's rhythms are singular, and Lean on Pete's long back half stretches on with the harsh unpredictability of the Oregon/Idaho/Wyoming landscapes the boy and the horse trudge across.

As the boy Charley (Charlie Plummer) gets lost, Haigh stirs in audiences a sense of lostness, too. He denies us our sense of certainty about where all this is headed, and Lean on Pete explores a world of racetracks and trailers and highway diners that the filmmakers just seem to have found rather than staged.